The present invention relates to an SAW (Surface Acoustic Wave) element which is composed of a plurality of SAW resonators, and an SAW device in which the SAW element is mounted, in particular to a technique effectively applicable to the SAW element of which a ladder-type circuit is constituted by these SAW resonators. Further, the present invention is also applied to a dual-type filter and a branching filter each of which is using a plurality of SAW elements. Furthermore, the present invention also relates to a technique effectively applicable in particular to reduction of coupling between a plurality of SAW elements.
In recent years, a mobile communication terminal equipment, such as a portable telephone, and the like has been rapidly progressed. From the viewpoint of convenience of portability or handiness, it is strongly desired that the mobile communication terminal equipment is not only small in size but also light in weight. In order to fabricate such a small and a light terminal equipment, it is essentially required that an electronic part or device used therein is also small in size and light in weight. Under the circumstances, many SAW elements, each of which has advantage for being fabricated small and light and each of in which a plurality of SAW resonators are formed in a piezo-electric substrate, have been used in a high-frequency section and an intermediate-frequency section of the terminal equipment.
An insertion loss and attenuation outside the pass band can be pointed out as important characteristics required for the SAW element, when the SAW element is used as a filter. Herein, the insertion loss influences power consumption of the mobile communication terminal equipment including the SAW element. The insertion loss becomes lower, the life of a battery included in the mobile communication terminal equipment becomes longer. Therefore, a capacity of the battery can be reduced, dependent on the insertion loss. Accordingly, the mobile communication terminal equipment can be fabricated small in size and light in weight, when the insertion loss can be low. Further, when wide range attenuation outside the pass band can be obtained in a filter, the mobile communication terminal equipment can also be fabricated small in size and light in weight.
In a known example of the SAW element, a serial arm is formed between input and output terminals while a plurality of parallel arms are formed between the serial arm and a reference potential terminal, and then SAW resonators are adequately located in these serial and parallel arms.
In such an SAW element, a resonant frequency of the SAW resonator located in the serial arm is adjusted to be corresponding with an anti-resonant frequency of the SAW resonator located in the parallel arm. Thereby, an input and output impedance is matched with a characteristic impedance around the frequencies corresponding with each other. The pass band is thereby constituted.
As regards an SAW element used as a band filter, it is strongly desired that the insertion loss is low and that amount of attenuation is enlarged in a frequency range outside the pass band.
Further, in a mounting of the SAW elements used in such an SAW device, it is carried out that an SAW element at the transmission side (a transmission filter) and an SAW element at the reception side (a reception filter) are mounted on a mounting substrate separately from each other. On the other hand, it is sometimes carried out that the transmission filter and the reception filter are mounted in a body on the mounting substrate. In these mountings, each of the transmission filter and the reception filter is mounted in the respective direction. As a result, due to the directions in which the transmission filter and the reception filter are mounted, coupling is sometimes caused to occur between the transmission filter and the reception filter. Consequently, a frequency characteristic of the SAW device is sometimes influenced by the coupling. Such an influence on the frequency characteristic due to the coupling is a serious problem regarding characteristics of the SAW device itself. It is therefore necessary to remove the influence as completely as possible.
As important characteristics required for the SAW device, it should be pointed out that an insertion loss is low and that amount of attenuation outside the pass band frequency is large. The insertion loss influences power consumption of the mobile communication terminal equipment. The insertion loss becomes lower, the life of a battery included in the mobile communication terminal equipment becomes longer. Therefore, a capacity of the battery can be reduced, dependent on the insertion loss. Accordingly, the mobile communication terminal equipment can be fabricated small in size and light in weight, when the insertion loss can be low. Further, when wide range attenuation outside the pass band can be obtained in an SAW device, the mobile communication terminal equipment can also be fabricated small in size and light in weight.
Herein, the SAW element used in the SAW device has a pair of exciting electrode portions each of which has a shape of the teeth of a comb and which are formed on a piezo-electric substrate with being involved in each other. With the structure, in the SAW element, an electric field generated by applying voltage between the exciting electrode portions produces a surface acoustic wave (SAW) on the piezo-electric substrate. On the other hand, a produced surface acoustic wave (SAW) is converted into an electric signal by the exciting electrode portions also in the SAW element. Herein, a film thickness of each exciting electrode portion is an important parameter for determining the pass band frequency as a filter. For example, in order that the pass band frequency may be enhanced from 1 GHz into 2 GHz, the film thickness 0.4 μm at the frequency 1 GHz must be reduced to a half of that, namely, the film thickness 0.2 μm at the frequency 2 GHz. Such reduction of the film thickness inevitably makes an area of section of a conductor pattern be small. As a result, an electric resistance value of the conductor pattern is increased. Accordingly, the above-mentioned insertion loss becomes high. Further, the amount of attenuation outside the pass band frequency cannot be obtained sufficiently. Thus, characteristics of the SAW element are deteriorated.